2Norwegian Institute for Air Research, Kjeller, Norway
3Department of Geosciences, University of Oslo, Norway
3NASA/Goddard Institute for Space Studies, New York, New York, USA
4Department of Environment and Geographical Sciences, Manchester Metropolitan University, United Kingdom
*now at: Bureau of Patents, Oslo, Norway
Abstract. Trends in cirrus cloud cover have been estimated based on 16 years of data from ISCCP (International Satellite Cloud Climatology Project). The results have been spatially correlated with aircraft density data to determine the changes in cirrus could cover due to aircraft traffic. Main emphasis has been on the area covered by the METEOSAT satellite, to avoid trends in the ISCCP data resulting from changing satellite positions. An alternative retrieval of high clouds in this region has been used to complement the analysis based on ISCCP data. In Europe, which is within the METEOSAT region, we find indications of a trend of about 2%/decade due to aircraft, in reasonable agreement with previous studies. The positive trend in cirrus in areas of high aircraft traffic seems to have contrasted a general negative trend in cirrus. Extrapolation in time to cover the entire period of aircraft operations and in space to cover the global scale yields a best estimate of 0.05 Wm−2 for the radiative forcing due to aircraft. This is close to the value given by IPCC (1999) as an upper limit.